The use of conversion coatings for the purpose of inhibiting corrosion of a metal substrate is well known. Phosphating is a widely used form of metal pretreatment. While phosphate coatings inhibit the corrosion of the metal substrate, the porosity of a phosphate coating results in only a limited measure of protection. For this reason, a phosphatizing process is often followed by a separate rinse process in order to provide for sealing of the pores of the phosphate coating. Chromic acid based solutions have often been used as such sealing rinses for phosphate coatings in the prior art. While chromic acid solutions as sealers are effective, due to the toxicity of the chromic acid solution waste disposal is difficult. Also, the strongly corrosive nature of a chromic acid solution complicates the storage, handling, and use of such solutions.
Sealants for phosphatized metal substrates which avoid the use of chromic acid are known in the art. For example, U.S. Pat. No. 3,196,039, Herbst et al, discloses a process and solution for sealing a phosphate coating on a metal surface. The solution contains a polyvinyl phosphonic acid and/or copolymers of vinyl phosphonic acid and/or acid derivatives thereof which contain, per monomer unit, only one free hydroxyl group at the phosphorus atom. Suitable components for the copolymer include mono or polyunsaturated organic compounds having a substantially polar character such as acrylic acid, methacrylic acid and the esters of aliphatic alcohols thereof, the amides and nitriles thereof, also vinyl esters such as vinyl acetate and vinyl propionate, maleic acid anhydride and crotonic acid. After treatment with the sealing solution, the metal parts are heated to from 80.degree. C to 180.degree. C to dry.
U.S. Pat. No. 4,220,485, Howell et al discloses a composition for sealing of phosphatized metal components which consists of a phosphoric acid, a zinc compound, a heavy metal accelerator and/or crystal refiner and a phosphonate corrosion inhibitor. While the sealant composition of Howell et al avoids he use of chromic acid, the inclusion of a heavy metal accelerator such as vanadium, titanium, zirconium, tungsten and molybdenum raises concerns regarding disposal of waste products. The composition disclosed in Howell et al includes a phosphonate as a corrosion inhibitor.
The use of a phosphonate as a corrosion inhibitor is known in the art. U.S. Pat. No. 4,501,667, Cook, discloses a corrosion control solution and process to inhibit the scale deposition from aqueous systems which comprises adding a 2-amino-phosphonoacetic acid compound to an aqueous system in contact with metal surfaces and as a preconditioner for metal surfaces prior to contact with a corrosive environment. In completely aqueous systems, such as cooling water systems, the Cook disclosure notes that further corrosion inhibitors can be added which may include methylamino- dimethylene-phosphonic acid employed in combination with the 2-amino-phosphonoacetic acid compound described. In the totally aqueous system described in Cook, the corrosion inhibiting additives are in continuous solution contact with the surface which is being treated.
U.S. Pat. No. 4,517,028, Lindert, discloses a treatment for metal surfaces which comprises contacting the surface with a polymer which is based on derivatives of poly-alkenylphenol polymer.
The use of a phosphonic acid complexed with a divalent metal in an aqueous solution for hot water or steam sealing of anodically produced oxide layers on aluminum is disclosed in U.S. Pat. No. 3,900,370, Germscheid et al. In the Germscheid patent the use of an aqueous solution of phosphonic acid in combination with calcium ions at a pH of from 5 to 6.5 as a sealant for an anodically produced oxide layer on aluminum is disclosed. The high temperature conditions disclosed for sealing of the electrolytic, anodic coating are not necessary in the practice of the present invention.
The advantages to be gained through the use of sealers to improve the corrosion resistance of conversion coatings has been recognized, as has the objectionable properties of typical chromium compounds employed as a sealer. The most commonly described non-chromium sealers, based upon amines, tannins, aminoalkylated polyvinylphenol and heavy metals have not earned wide acceptance, principally due to disappointing performance in retarding corrosion. Furthermore, some of the heavy metal based sealers may pose significant waste disposal problems.